What is the one test of greatest predictive value for the fetus? Afp?

Prediction? Predictive value for what? A high AFP is associated with a fetus with a neural tube defect. A large nuchal translucency is associated with chromosomal abnormalities and cardiac defects. A normal level ii ultrasound reduces the risk of structural abnormalities in the fetus. So it depends upon what you are testing for.
PAPP-A. I presume you are referring to the predictive value of maternal serum analytes in predicting dire perinatal complications. As such, I would rank pregnancy-associated placental protein-a (papp_a) as the most important one because it can predict poor fetal growth and increased risk for perinatal death months in advance.